A number of recently developed techniques and emerging standards are based on employing multiple antennas at a base station to improve the reliability of data communication over wireless media without compromising the effective data rate of the wireless systems. Specifically, recent advances in wireless communications have demonstrated that by jointly encoding symbols over time and transmit antennas at a base station, one can obtain reliability (diversity) benefits as well as increases in the effective data rate from the base station to each cellular user. These multiplexing (throughput) gain and diversity benefits depend on the space-time coding techniques employed at the base station. The multiplexing gains and diversity benefits are also inherently dependent on the number of transmit and receive antennas in the system being deployed, in the sense that they are fundamentally limited by the multiplexing-diversity trade-offs curves that are dictated by the number of transmit and the number of receive antennas in the system.
In many emerging and future radio networks, the data for any particular cell user may be available to multiple base stations. Viewing each of the base stations with data for a particular user as an element of a virtual antenna array suggests using cooperative signal encoding schemes across these base stations to provide diversity benefits to the desired user. Thus, when transmitting the same data from two active base stations, each active base station must encode its data independently. Since the encoded signals, however, are transmitted by spatially dispersed base-stations, they arrive at the receiver with distinct relative delays with one another, i.e., asynchronously. That is, there can be a lack of time-synchronization between the transmissions from different base stations to the receiver. This asynchrony can arise due to the fact that the individual base stations may be operating independently, but also due to the fact that even if the signals the signals transmitted from spatially dispersed base stations to a receiver are transmitted synchronously, they may arrive asynchronously at the receiver. Although these relative delays can, in principle, be estimated at the receiver, they are not known (and thus cannot be adjusted for) at the transmitting base stations, unless there is relative-delay information feedback from the receiver to the transmitting base stations.
A large collection of space time block codes (STBCs) have been proposed in recent years as a way of providing diversity and/or multiplexing benefits by exploiting multiple transmit antennas in the forward link of cellular systems. Given the presence of n transmit antennas, the typical objective is to design STBCs that provide order-“n” transmit-antenna diversity in the system. Typical STBC designs transmit an antenna-specific block of t samples per antenna for each block of k information symbols. Such STBC designs are described by a STBC matrix with t rows and n columns, whereby the (i,j)th entry denotes the sample transmitted by the antenna j at time i. Of interest is the actual symbol rate of the STBC scheme, R, which is equal to k/t (i.e., the ratio of k over t). Full rate STBCs are STBCs whose rate R equals 1 symbol per channel use. Another important attribute of a STBC is its decoding complexity.
Existing orthogonal space-time block codes that have been designed and optimized for the synchronous setting perform poorly, in general, in the asynchronous non-collocated antennas setting. For instance, it is well known that the Alamouti code is the best space-time block code for a two-transmit one-receive antenna system. Although the Alamouti code provides full rate, full diversity, maximum coding gain, and symbol-by-symbol decoding in the synchronous setting, it provides no spatial diversity in the case that there is a one-sample relative delay offset at the receiver between the signals transmitted by the two base-stations. This observation is more general, in that, codes that have been optimized for the synchronous setting perform, in general, poorly in the presence of relative delay offsets in the arrival times of signals transmitted from different base stations. For more information on Alamouti codes, see S. M. Alamouti, “A simple transmitter diversity scheme for wireless communications,” IEEE Journal Selected Areas in Communications, pp. 1451-1458, October 1998.
One class of designs for transmitting to a receiver from n potentially asynchronous base stations has been presented in S. Wei, D. Goeckel, and M. Valenti, “Asynchronous cooperative diversity,” Proceedings Conf. Information Sciences and Systems, Princeton University, March 2004. The proposed method employs space-time block coding with BPSK modulation without reducing the symbol rate. The resulting full-rate schemes rely on the presence of a Viterbi decoder at the receiver in order to be able to provide full diversity at the receiver regardless of the delay offset, provided however that it does not exceed a predetermined value L. Despite being a full-rate full-diversity scheme, this technique inherently possesses a number of important limitations. First, the data block size and the decoding complexity of the proposed algorithms, required to obtain full spatial diversity are both exponential in the delay parameter, L; hence, the decoding complexity and delay become prohibitively expensive fast as the number of transmit base stations increases. Second, although full rate in the sense that 1 symbol is communicated per channel use, these schemes only work with BPSK modulation (which, being a real-valued modulation scheme, uses only half of the available degrees of freedom); as a result, the rate of these schemes is equivalent to that of half-rate QPSK schemes (which use both dimensions in the complex plane to modulate symbols).
Another class of approaches exploits space-time trellis code designs that are designed to provide full diversity regardless of the relative delay subject to maximum allowable relative delay offset, L. These codes exploit constructions based on shift-full rank matrices that guarantee that the matrices provide full diversity regardless of the relative delay set. Although they provide full diversity subject to a set of relative delays with only a small overhead in data rate, these designs have some important limitations. First, the decoding complexity of these designs is exponential both in the number of antennas, and the parameter L. Second, the design is modulation scheme specific. Finally, they are guaranteed to provide full diversity subject to relative delay offsets that are integer multiples of the symbol duration. Strictly speaking, however, there are no guarantees for relative delays that are fractions of the symbol duration.
Although the decoding complexity of the optimal decoder for arbitrary STBCs is exponential in the number k of jointly encoded symbols, there exist designs with much lower complexity. One such attractive class of designs, referred to as orthogonal space-time codes (OSTBCs), can provide full diversity while their optimal decoding decouples to (linear processing followed by) symbol-by-symbol decoding. Full rate OSTBCs exist only for a two transmit-antenna system. For three or more antennas, the rate cannot exceed ¾ symbols/per channel use. This rate is achievable for n=3 and n=4 antennas. For more than four antennas the highest-rate OSTBCs are not known in general. In general, a rate equal to ½ symbols/channel use is always achievable, but, often, higher rates may also attainable for specific values of n.
Classes of systems that have been employed in Europe for broadcasting common audio/video information from several base stations are exploiting coded OFDM transmission under the umbrella of the single frequency network (SFN) concept (e.g., see J. H. STOTT, J. H., 1996. “The DVB terrestrial (DVB-T) specification and its implementation in a practical modem,” Proceedings of the 1996 International Broadcasting Convention, IEE Conference Publication No. 428, pp. 255-260, September 1996). These systems employ a common coded OFDM based transmission from each of the broadcasting base-stations. The OFDM based transmission allows asynchronous reception of the multitude of signals and provides increased coverage. However, as all base-stations transmit the same coded version of the information-bearing signal, SFN systems do not provide in general full transmit base-station diversity (some form of it is available as multi-path diversity, although limited).
A class of schemes includes space-time bit-interleaved coded modulation systems with OFDM and can provide both spatial diversity and can cope with asynchronous transmission. Although these schemes can provide full diversity and very good data rates, they are disadvantageous because the decoder complexity of such schemes grows exponentially with the number of transmit antennas used over all the base stations, and the number of bits/per symbol in the employed modulation scheme.